So basically what the title says.
For me it would have to be in Plutonia (forgot which map it was) when an Arcvile kept on resurrecting a Baron of hell in a narrow hallway, after countless times of killing the Baron I was able to get pass him and unleashing the BFG on the Archvile.

When I decided to do a keyboard-only run through Doom II on Ultra-Violence one day. I blasted through the first 8 levels and arrived to Map09; usually I like that level, but this time I kept getting killed so many times it wasn't funny at all, and I refused to cheat. After 30+ reloads I started naming my savegames "i hate doom 2 so much", "fuck you doom 2", "this map needs to die" and so on.

After a long time I got to MAP10, but accidentally overwrote my savegame while I had 2% health, and spent the following 10 minutes trying to survive but getting killed over and over. I got so fed up with that, I had to quit.

On my run of Doom2 I had reached map24, The Chasm. Died there more than any other place in the mapset, and at least twice to the nukage pits in that second to last room to the exit, only because those pits are inescapable. Overall its my least favorite map.

Probably not the most frustrating, but certainly worth mentioning, is trying to beat MAP30 from The Plutonia Experiment with -fast. It took me ages to finish the previous 29 maps, and I could never finish that one.

Mine would be dealing with revenants throughout Plutonia in general. Can't remember now which map it was, but there is a room whose walls open from all directions, revealing revenants all around. This moment took me ages to finish, even with save game abusing.

I have a infuriating habit of mixing up the quick save and quick load menus.

I try to avoid save scumming so when I save it's usually after a tough fight so you can imagine my annoyance when I instead accidentally load my previous save from before the fight takes place. Even worse when I die on a tough map and instead of loading the save state just prior to my death I accidentally overwrite my save game at the start of the level with a pistol start.

Surprise Revenants or Arch-Vile hoards from nowhere with no cover. I hardly ever save, so these usually mean I'm restarting the map from scratch. They're especially annoying when the map hasn't been difficult at all beforehand, because I'm even less likely to have a save then.

Monsters placed under (or over) platforming sections in vanilla maps, so you hit invisible walls as you try to make your jumps.

Blue Shadow said:
MAP09 of Plutonia (Abattoire); to be more specific, the exit area. I just despise that part.

I have just watched a video of this level to see what you meant and it turned out that it's the same area I had referred to in my post! This one sure is nasty. However, there are "only" a few revenants there, accompanied by mancubi. Doesn't change a lot really, because it's still hard as hell.

After Doom 4 comes out when I start seeing people whining about how it wasn't some dream god-tier masterpiece exactly catered towards their personal vision of how Doom 4 should've been, when it will still probably be a pretty good game, if not amazing like D3 which received similiar treatment nonetheless.

Until then, seeing people complain that Doom 3 wasn't a Painkiller clone.

Overall disappointment with Doom, no really awesome releases last year (all "big" ones seemed "just quality" at best). I wonder if my love for Doom is dead and it will never be as exciting as it was. But I'll wait some more before abandoning all hope.

Anything to do with puzzles in any Doom WAD or IWAD. I came to play Doom to kick ass and take names, not get stuck for five minutes wondering what the hell to do and where the hell to go. (I'm looking at you, Map06 of Whitemare...)

Hectic said:
Almost beating a map (usually large or a new map) and getting killed, or when GZDoom crashes in the same situation. I usually "rage quit" as a result.

Tell me about it. I have recently almost finished Deus Vult map05 without dying. Got killed after 2h or so, very close to the end.

Antroid said:
After Doom 4 comes out when I start seeing people whining about how it wasn't some dream god-tier masterpiece exactly catered towards their personal vision of how Doom 4 should've been, when it will still probably be a pretty good game, if not amazing like D3 which received similiar treatment nonetheless.

Until then, seeing people complain that Doom 3 wasn't a Painkiller clone.

Pressing the spacebar as I die. It infuriates me. I'm not even sure why, it's not like I enjoy long death sequences in other video games. But then, perhaps there's a right balance. Skip the death entirely, and it feels... weightless? I guess I like the ability to restart as soon as I want, but I'd like to be in control and not restart sooner than I want.

That happened in a demo I recorded for myself the other day. Tried to open a door as I was getting sprayed by a chaingunner, and the time between dying, pressing the use key, and starting the level over made me feel as though I'd accidentally activated a teleporter that brought me to the start of the level.

It's kinda like if you were to hand in a test to a professor and without even grading it, just throws it back at you and tells you to start over.

Inkie said:
On my run of Doom2 I had reached map24, The Chasm. Died there more than any other place in the mapset, and at least twice to the nukage pits in that second to last room to the exit, only because those pits are inescapable. Overall its my least favorite map.

hehe, probably this. i gave up here on my first journey through doom2, and can't stand maps where one can fall from ledges and into lava etc to this day ;)

got "time sucks" on my first playthrough of tnt'`s map 20 or 21, one of these two huge map.

Probably the Icon of Sin, as I still haven't beat it without mouselook, mods, or cheating. I don't want to try.

Another moment, while not Doom related, was in Dark Forces II where I couldn't figure out how to open doors that didn't have a switch next to them. THe first level had three doors each with switches, then suddenly throws one at you with no visible switch. Took me months actually to figure out that I could just press Use on the door itself. If I had played Doom first, maybe this could've been avoided, as the first Doors don't activate with a switch. If they did, well, who knows.